Machine shops universally use end mills in computerized milling machines, e.g. computer numerical control (CNC) milling machines, as a cutting tool for all industrial milling applications, such as profile milling, tracer milling, face milling, and plunging. End mills are categorized by the number of flutes, the helix angle, the material and the coating material. Over the last two decades the speed of CNC milling machines has increased ten fold, requiring the use of milling cutters with exotic coatings or made from solid carbide to improve part cycle time and tool life. Moreover, contemporary tools have more aggressive geometry, are more expensive than regular high-speed steel cutters, and are more difficult to recondition. Performance end mill type milling cutters used in the mainstream production job shop market are made from the most premium grades of solid carbide, and are rotating at spindle speeds much higher than ever before, requiring different structural and performance geometry with high finish and accuracy. A high-level of attention must be given to tool features, tolerances and finish or they will not cut and extract material with tolerable heat and friction at higher speeds.
Cemented tungsten carbide tools consist of a composition of tungsten carbide powder with cobalt binder. The binder breaks down over time due to cycling of load moments, causing the leaching of the cobalt away from the carbide powder. This leads to localized degradation of the integrity of the substrate to such a degree that the area of the end mill cutter tool that was under the highest loads should not be returned to service as the material has been altered at the inter-granular level. In addition, current protocols, such as ISO9000, prescribe that if an end mill cutter diameter is altered requiring an edit to the program speeds, feeds and offsets used to run the part, the part is required to be re-inspected prior to commencing production. Re-inspection slows down the manufacturing/shop process and is therefore unwelcome. Also, the end mill cutter tool that has just been returned to service generally does not perform as well as or as long as a new one.
Even with good equipment, an experienced technician with a keen eye and magnification is required to recondition end mill cutting tools. However, careless reconditioning can result in disproportionately maintained clearance angles and flute geometry, affecting friction and chip evacuation and causing a reduction in performance.
In small carbide milling cutters, the area of high exposure to heat, load and friction is a “throw away” or for one time use, due to material breakdown. As a general rule, tools that can be ground back into areas free of substrate breakdown can be effectively re-sharpened with shop support equipment. In general, shops do not use reconditioned tools because they are usually small and associated with too many issues, which affect the true reconditioning cost.
High loads combined with more radical tool geometry, and stringent concentricity requirements mean new carbide end-mill cutter tools, as a rule, must be held more accurately with greater force during the machining process than their high-speed steel predecessors. Carbide end-mill tools are also used differently in shops today, wherein faces on parts are often machined with stub flute tools having solid reduced necks giving greater stability at higher material removal rates, although long flutes are still the method of choice for finishing larger faces with high finish.
To sharpen carbide end-mill cutters, the end is cut off to get into new, unused material, while watching to ensure the tool does not over heat. Furthermore, special care must be taken to ensure: the diameter does not change; a flat or “notch” is produced, so that the tool can be held well in a side lock tool holder; a reduced neck is ground; a ball nose is resharpened; and a corner rad “bull nose” champher or blend radius is produced.
The principal attraction of conventional bench top sharpeners, such as those produced by Cuttermaster®, Darex®, and Chevalier®, has been cost, and a perceived ease of use. Moreover, they have been servicing a market in which High Speed Steel (HSS) tools were being used in an environment with mostly conventional or CNC machines having spindles designed to operate below 5000 rpm that would tolerate a reground cutter, i.e. rotational speeds and part feeds were lower, with less pressure on part cycle.